1. Field of the Invention
The present invention relates to a brushless motor system driven by using a direct current (DC) power source, and more particularly to a brushless motor system designed to be capable of considering a counterplan to electromagnetic noise.
2. Description of the Related Art
A brushless motor having neither a brush nor a commutator is free from wear because of including no mechanical contacts. Therefore, the brushless motor has advantages, such as low noise, high-speed rotation and long life, and is becoming increasingly prevalent. In the brushless motor, a rotating magnetic field is generated by controlling a current flowing through each wire (motor winding) wound over a stator, and a mechanical rotatory power is given to a rotor by the produced rotating magnetic field. An inverter is required to control the current flowing through the motor winding. Accordingly, it can be said that there are three basic components for a brushless motor system, i.e., the power source, the brushless motor, and the inverter.
A brushless motor loaded on a vehicle requires to be controlled with high accuracy. For that reason, PWM (Pulse Width Modulation) control using an inverter is performed in many cases. In the PWM control, high-speed switching is repeated by semiconductor switching devices in the inverter. The repeated high-speed switching causes abrupt potential variations in the motor winding. Because of the presence of stray capacitance between the motor winding and a metal housing of the motor, electric charges are repeatedly charged to and discharged from the stray capacitance with the repeated switching by the semiconductor switching devices.
Usually, because the metal housing of the motor is directly contacted with or indirectly connected to a ground, currents charged to and discharged from the stray capacitance with the repeated switching in the PWM control are generated. As a result, those charge and discharge currents are leaked as noise currents to the ground. The noise currents leaking to the ground flow in common to all the motor windings and therefore are called a common mode current. The common mode current is a main factor of electromagnetic noise.
FIG. 2 is a reference view showing main paths along which the common mode current flows in a vehicle-loaded brushless motor system driven by a DC power source, for example, when no measures are taken against noise. The common mode current starts to flow from an inverter 200 and is leaked to a ground 108 through stray capacitance 201 existing between each motor winding 104 and a metal housing 105 of the motor. The leaked common mode current is returned to the inverter 200 through a ground line and a power source line of the inverter 200, thereby forming loops 202 and 203 shown in FIG. 2.
The common mode current not only flows, as conduction noise, into the system itself, peripheral devices and so on, but also generates radiation noise while flowing along the large loops shown in FIG. 2. In other words, the common mode current is a main factor causing the conduction noise and the radiation noise, thus giving rise to adverse influences such as radio noise and malfunction.
Generally, a filter circuit is disposed in the inverter to reduce the electromagnetic noise. For example, Patent Document 1 (JP-A-8-47244) discloses a system in which a filter circuit is disposed in each of the power source side and the motor side of an inverter circuit.
Switching at a higher speed in the PWM control has been recently proposed with intent to increase the speed and accuracy in control of the brushless motor, as well as to reduce a switching loss in the inverter for realizing higher efficiency. Steeper changes of voltage and current caused by the higher-speed switching increases a noise current. Also, a larger current flowing through the inverter with a higher output of the motor increases the noise current. Thus, adverse influences of electromagnetic noise upon the brushless motor system itself and the peripheral devices become more serious.